In general, fungicides have proven to be most effective in controlling foliar diseases on wheat in Kansas when applied in a single application sometime around flag leaf to head emergence stages, depending on level of disease risk (disease pressure and predicted weather conditions), variety resistance to the most threatening fungal diseases, yield potential, foliar fungicide efficacy, and other factors.
Fungicides can also be applied in a split application, with an early application made around early spring greenup followed by a later application at flag leaf to early heading stage. That approach adds a little extra expense, and may or may not pay off compared to the single application approach, as the majority of the yield response is normally associated with the flag leaf application. It is also important to remember that fungicides will only protect the leaves present at time of application; thus, an application during jointing does not substitute for a flag leaf application, as any leaf that emerged after the application will not be protected.
When making split applications, the early application often uses a low rate of product to save money. This provides a shorter length of control than a full-rate application. With the prevalence of low-cost generic fungicides on the market now, some producers are using a full rate of fungicide for the early application. The full-rate of most fungicides provides about two weeks of good protection, followed by a third week of partial protection to the leaves present at the time of application. Using a full rate early, however, could have implications for the second, later application. Growers will need to select a product and rate that stays within the labeled limits on the amount of each active ingredient used in a single season. You don’t want an early fungicide application to remove the ability to apply your preferred product at flag leaf.
Advantages and limitations of split applications
There are some advantages to making an early application, and some limitations. The advantages of early-season fungicide application include:
The limitations of early-season fungicide application:
K-State test results of early, low-rate fungicide applications indicate this practice is most likely to be effective in continuous wheat grown in high-residue conditions, and with varieties that are susceptible to either tan spot or powdery mildew. The value of the early applications is diminished in other rotations, conventional tillage systems, or with a variety that is moderately resistant or resistant to the targeted disease – usually tan spot or septoria leaf blotch, and possibly stripe rust or powdery mildew some years. K-State has not tested the practice of making split applications using a full-rate of product at both times.
Which diseases are present early?
Stripe rust and leaf rust rarely overwinter in Kansas. These diseases typically blow into the state from Texas and Oklahoma during the spring, and often do not begin to cause significant infections on wheat until about flag leaf emergence at the earliest.
However, stripe rust (Fig. 1) bears watching. It is primarily a cool-weather disease and starts a little earlier in the season than leaf rust. Stripe rust infections can quickly become severe and damage leaves. Leaf rust does not act quite as quickly, nor does it damage leaves as readily as stripe rust. Tan spot and septoria leaf blotch (Figs. 2 and 3), can also damage leaves quickly and may need to be controlled early if present in a field. Powdery mildew can become established early in a growing season, but this disease does not normally cause severe yield losses in Kansas, except on varieties that are very susceptible.
If a field has some hot spots of stripe rust at jointing or earlier, that would be an additional factor to take into consideration when deciding whether a split application of fungicide would be helpful. The same applies to tan spot, septoria leaf blotch or leaf rust. An early infection of or powdery mildew, however, would not be as important in making that decision.
Figure 1. Symptoms of stripe rust on wheat. Notice the blister-like lesions arranged in stripes. Photos by Erick DeWolf, K-State Research and Extension.
Figure 2. Symptoms of tan spot on wheat. Lesions are tan, with yellow margins, and mature lesions often have a darkened spot in the center. Photos by Erick DeWolf, K-State Research and Extension.
Figure 3. Symptoms of septoria leaf blotch. Lesions are tan, elongated, with thin yellow margins. Black speckles in the center are key identifying features. Photos by Romulo Lollato, K-State Research and Extension.
Product rates and restrictions
Producers considering the use of split applications must pay close attention to label restrictions. Every active ingredient in a fungicide has a maximum total amount that can be applied during the season.
For example, if an early application of a generic form of tebuconazole is applied at 4 oz/acre, a subsequent application of any fungicide containing tebuconazole around heading would put you over the 4 oz limit for the crop season. In addition, a subsequent application of a fungicide premix that contained tebuconazole would put you over the limit.
Thus, be sure to read the label to determine the maximum amount of a chemical that can be applied in a single season and the exact amount of a chemical(s) that is in a fungicide.
For information the efficacy of different foliar fungicide products, refer to K-State Research and Extension publication Foliar Fungicide Efficacy Ratings for Wheat Disease Management 2016, EP130.
The main conclusions we can draw from recent studies in Kansas and Oklahoma:
For information on disease susceptibility of wheat varieties, see K-State Research and Extension publication Wheat Variety Disease and Insect Ratings 2016, MF991.
For information on assessing the need for wheat foliar fungicide, refer to K-State Research and Extension publication Evaluating the Need for Wheat Foliar Fungicides, MF3057.
Another publication providing good information, from which a few excerpts were used in this article, is Oklahoma State University’s Split Versus Single Applications of Fungicides to Control Foliar Wheat Diseases, PSS-2138.
Erick DeWolf, Extension Plant Pathologist
Romulo Lollato, Wheat and Forages Specialist